NBC NATIONAL NEWS — New York City Police and the FBI are investigating a false threat in which a caller claimed a hijacker was hiding in a plane set to land at John F. Kennedy Airport Monday afternoon.

The threat was phoned in between 3 to 3:30 p.m., just as American Airlines Flight 24 from San Francisco and Finnair Flight 5 from Helsinki were about to land at Kennedy, according to law enforcement sources.

"The captain got on and he said, 'I'm not sure if this is bogus or not, but there's going to be some law enforcement people coming on the plane,'" passenger Tim Skelly recounted.

When the planes landed at JFK, they were moved to an isolated area to be searched.

Sources said the level of concern was low, but audio recordings between the pilot of one of the planes and the control tower at Kennedy Airport reveal a tense exchange as they both attempted to figure out why the plane was being met by emergency vehicles.

"Can you contact your company? Do they have any information for you?" the air traffic controller asks the pilot in the audio recordings.

"Negative, they're not answering. What do you have?" the pilot asks.

"I don't have a thing at this moment, except that you and the aircraft beside you need to wait in that area," the controller says.

The pilot says, "OK, we're surrounded by emergency vehicles. There's a reason for this. Somebody's got to give us the reason, or we're going to evacuate the aircraft. You got 60 seconds."

Finally, the controller tells the pilot, "We have the information. Can you possibly call?"

The pilot responds, "Negative. I would demand the information right now over a frequency."

The Port Authority Police received a phoned threat that hijackers wearing gas masks were hiding in the wheel wells of the American Airlines plane, but the information was vague, sources said.

The plane was carrying four federal air marshals.

"They told us to stay calm while they searched the plane, but it was very nerve-wracking," said passenger Debbie Teitelbaum.

The Finnair flight was cleared around 4:30 p.m., a federal official said, and an all-clear on the American Airlines flight followed shortly afterward.

All wheel wells of both planes were checked and police found nothing, sources said.

Neither pilot indicated an emergency.

Police and FBI interviewed passengers and crew on both planes as part of the investigation, and are reviewing the phone call in which the threat was made.